Revisiting the statistical equilibrium of H${}^{-}$ in stellar atmospheres

Abstract

The negative hydrogen ion H${}^{-}$ is, almost without exception, treated in local thermodynamic equilibrium (LTE) in the modelling of F, G, and K stars, where it is the dominant opacity source in the visual spectral region. This assumption rests in practice on a study from the 1960s. Since that work, knowledge of relevant atomic processes and theoretical calculations of stellar atmospheres and their spectra have advanced significantly, but this question has not been reexamined. We present calculations based on a slightly modified analytical model that includes H, H${}_2$, and H${}^{-}$, together with modern atomic data and a grid of 1D LTE theoretical stellar atmosphere models with stellar parameters ranging from T${}_{\mathrm{eff}}=4000$ to 7000~K, $\log g=1$ to 5 cm/s${}^2$, and [Fe/H]$=-3$ to 0. We find direct non-LTE effects on populations in spectrum-forming regions, continua, and spectral lines of about 1-2% in stars with higher T${}_{\mathrm{eff}}$ and/or lower $\log g$. Effects in models for solar parameters are smaller by a factor of 10, about 0.1-0.2%, and are practically absent in models with lower T${}_{\mathrm{eff}}$ and/or higher $\log g$. These departures from LTE found in our calculations originate from the radiative recombination of electrons with hydrogen to form H${}^{-}$ exceeding photodetachment, that is, overrecombination. Modern atomic data are not a source of significant differences compared to the previous work, although detailed data for processes on H${}_2$ resolved with vibrational and rotational states provide a more complete and complex picture of the role of H${}_2$ in the equilibrium of H${}^{-}$. In the context of modern studies of stellar spectra at the percent level, our results suggest that this question requires further attention, including a more extensive reaction network, and indirect effects due to non-LTE electron populations.

arXiv commentAccepted by Astronomy and Astrophysics. Replaced with new version following language editing, as well as correct font, and correction of some minor typos

References in corpus (29)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• A new extensive library of PHOENIX stellar atmospheres and synthetic spectra
• The Formation of the First Star in the Universe
• Simulations of stellar convection with CO5BOLD
• The Stagger-grid: A Grid of 3D Stellar Atmosphere Models - I. Methods and General Properties
• Both accurate and precise gf-values for FeII lines
• Accuracy and precision of industrial stellar abundances
• Carbon, oxygen, and iron abundances in disk and halo stars. Implications of 3D non-LTE spectral line formation
• The Dawn of Chemistry
• Inelastic H+Li and H^-+Li^+ collisions and non-LTE Li I line formation in stellar atmospheres
• High-precision stellar abundances of the elements - methods and applications
• On inelastic hydrogen atom collisions in stellar atmospheres
• The GALAH Survey: Non-LTE departure coefficients for large spectroscopic surveys
• How realistic are solar model atmospheres?
• NLTE solar irradiance modeling with the COSI code
• Effective temperature determinations of late-type stars based on 3D non-LTE Balmer line formation
• Effects of Varying the Three-Body Molecular Hydrogen Formation Rate in Primordial Star Formation
• Rate of three-body recombination of hydrogen molecules during primordial star formation
• A NLTE line blanketed model of a solar type star
• Non-LTE Balmer line formation in late-type spectra: Effects of atomic processes involving hydrogen atoms
• 3D non-LTE iron abundances in FG-type dwarfs
• Non-LTE modeling of the near UV band of late-type stars
• Excitation and charge transfer in low-energy hydrogen atom collisions with neutral iron
• Atmospheric models of red giants with massive scale NLTE
• Sturmian theory of three-body recombination: application to the formation of H${}_2$ in primordial gas
• Titans metal-poor reference stars. I. Accurate effective temperatures and surface gravities for dwarfs and subgiants from 3D non-LTE H$\alpha$ profiles and Gaia parallaxes
• 3D Stagger model atmospheres with FreeEOS I. Exploring the impact of microphysics on the Sun
• C-type shock modelling -- the effect of new H${}_2$-H collisional rate coefficients
• Jacobian-Free Newton-Krylov method for multilevel NLTE radiative transfer problems